KR20050089086A - Variable numerical aperture large-field unit-magnification projection system - Google Patents

Abstract

An optical system for projection photolithography is disclosed. The optical system is a modified Dyson system capable of imaging a large field over both a narrow and a broad spectral range. The optical system includes a positive lens group having a positive subgroup of elements that includes at least a piano-convex element and a negative subgroup that includes at least a negative meniscus element. The lens subgroups are separated by a small air space. The positive and negative subgroups constitute a main lens group arranged adjacent to but spaced apart from a concave mirror along the mirror axis. The system also includes a variable aperture stop so that the system has a variable NA. A projection photolithography system that employs the optical system is also disclosed.

Description

Variable numerical aperture large field unit enlarged projection system {VARIABLE NUMERICAL APERTURE LARGE-FIELD UNIT-MAGNIFICATION PROJECTION SYSTEM}

FIELD OF THE INVENTION The present invention relates to optical projection systems, and more particularly to a variable numerical aperture, a large-field unit-magnification projection optical system.

Photolithography is currently used not only in submicron resolution integrated circuit manufacturing, but also in advanced wafer-level IC packaging and semiconductors, microelectromechanical systems (MEMS), nanotechnology (forming nanoscale structures and devices) and other applications. have. These applications require multiple imaging capabilities ranging from relatively low resolution (ie, resolutions of several microns) with large focal depths to sub-micron resolutions and high throughput.

 The invention, which is disclosed in the following detailed description of the invention, is incorporated by reference in the context of US Patent No. 4,391,494, incorporated by reference by Ronald S. Hershel, issued July 5, 1983 and issued to General Signals. Hereinafter referred to as the '494 patent).

1 is a cross-sectional view of an exemplary optical system 8 of the prior art according to the '494 patent. The optical system disclosed in this' 494 patent and shown in FIG. 1 is a unit-magnification that uses complementary and reflective elements to achieve large field sizes and large apertures (NA). ), Catadioptric, achromatic and astigmatic optical systems. The system is basically symmetrical about an aperature stop located in the mirror, eliminating coma, distortion and odd order aberrations such as lateral color. All spherical surfaces are nearly concentric and the center of curvature is located close to the area where the focal plane is located if the system is not folded. Thus, the system thus created is necessarily independent of the refractive index of the air layer in the lenses, thereby eliminating the need for pressure compensation.

The optical system 8 comprises a concave spherical mirror 10, an aperture stop AS1 located therein and a composite, achromatic plano-convex doublet lens-prism assembly 12 ) This mirror 10 and assembly 12 are arranged symmetrically with respect to the optical axis 14. The optical system 8 is necessarily symmetrical with respect to the aperture stop AS1 located in the mirror 10 whereby the coma, distortion and lateral color of the system are initially corrected. All spherical surfaces in the optical system 8 are nearly concentric.

In the optical system 8, the doublet prism assembly 12 includes a meniscus lens 13A, a flat iron lens 13B and a symmetric fold prism 15A, 15B. . This assembly 12 together with the mirror 10 corrects for residual optical aberrations including axial color, astigmatism, petzval and spherical aberration. Symmetrical folding prisms 15A, 15B are used to obtain sufficient working space for the movement of the reticle 16 and the wafer 16.

The optical system 8 also includes an object plane OP1 and an image plane IP1 that are separated through the folding prisms 15A, 15B. Larger workspaces reduce available field size by about 25% to 35% of the total potential field. In the past, this field size reduction was not important because it was possible to obtain both the required resolution and acceptable field size in modern circuits.

However, most current (next generation) high technology microfabrication processes (eg, wafer level IC packaging, semiconductor fabrication, MEMS and nanostructure formation, etc.) perform multiple exposure steps using 200 mm wafers and 300 mm wafers. It includes a process for doing so. In addition, the exposure steps must be performed in a manner that yields a large amount of throughput so that this manufacturing process is economically feasible.

Unfortunately, the '494 patented optical system provides high quality imaging at large field sizes (such as three to six 34 × 26 mm step and scan fields) with minimal resolutions ranging from 0.75 microns to 1.4 microns. Not capable This performance is, among other things, needed in so-called "mix-and-match" applications, where different masks that require different resolutions (typically requiring different photolithographic systems) are required for the microdevice manufacturing process. Used in

Summary of the Invention

The present invention is a variable NA, unitary magnification projection optical system. In one embodiment, the system is able to image at least six 34 mm by 26 mm step and scan fields, typically with a low NA configuration (ie, having a numerical value of 0.16 or less), and a high NA configuration (ie, At least three 34 mm by 26 mm step-and-scan fields with NA values greater than 0.34, and four or five with a mid-range NA configuration (i.e., NA with a size of 0.16 to 0.34). A 34 mm by 26 mm step and scan field can be imaged.

A first aspect of the invention is an optical system comprising a concave spherical mirror along an optical axis. A variable aperture stop located in the mirror determines the NA of this system. The main lens group with positive refractive power is located adjacent to the mirror and spaced apart from the mirror. This main lens group comprises a first subgroup having positive refractive power and a second subgroup having negative refractive power in order towards the mirror. This second subgroup is spaced apart from the first subgroup by an air layer. The system also includes a first prism and a second prism, each of which includes a first flat surface and a second flat surface. The second flat surfaces are disposed adjacent to the positive subgroup on opposite sides of the optical axis. The first flat surfaces are respectively disposed adjacent to the object plane and the image plane. The system also has an adjustable field size in the image plane by adjusting the variable aperture stop. This adjustable field size can image more than two 34 mm by 26 mm step and scan fields.

A second aspect of the present invention is a photolithography system comprising the projection optical system of the present invention.

Brief description of the drawings

1 is a cross-sectional view of an exemplary prior art unit magnification projection optical system according to the '494 patent.

2 is a cross-sectional view of a first exemplary embodiment of a unit magnification projection optical system according to the present invention.

3 is a cross-sectional view of a second exemplary embodiment of a unit magnification projection optical system according to the present invention.

4 is a cross-sectional view of a third exemplary embodiment of a unit magnification projection optical system according to the present invention.

5 is a schematic diagram of a photolithography system using a unitary magnification projection optical system according to the present invention.

The various elements shown in the figures are merely representative and are not drawn to scale. Thus, some parts of these elements can be enlarged and others can be reduced. This figure is intended to illustrate various implementations that will be apparent to those skilled in the art and that can be executed appropriately.

The present invention is a large field, light spectral band or narrow spectral band, color correction and astigmatism correction projection optical system for projecting an image of a pattern formed on a reticle onto a substrate (wafer) in a substantially unitary magnification manner. The unit magnification projection optical system of the present invention is an improved system of the prior art optical system of the '494 patent, which is the embodiment briefly described above with reference to FIG.

As used herein, the term "large field" means an exposure field having n rectangular step and scan fields ("n-tuple" for n = 2) each of which has a size of 34 mm x 26 mm. Thus, when this term is used herein, "exposure field" means the field size that a stepper system can image in a step and repeat mode of operation. The term “light spectral band” also refers to a spectral band comprising g, h, i mercury spectral lines (ie, 436 nm, 405 nm, 365 nm, respectively). The term “narrow spectral band” refers to a spectral band comprising i lines (eg, 365 nm) ± 10 nm.

The projection optical system of the present invention described in detail below has very good image quality (i.e., multi-color strel ratio of 0.96 or more) over a large field and light spectral band or narrow spectral band.

The projection optical system of the present invention has two preferred configurations, a light spectral band low NA configuration and a narrow spectral band high NA configuration. The broad spectral band low NA configuration is particularly useful in wafer level IC packaging applications such as bump lithography which does benefit from relatively high radiation imaging and does not require the highest level of resolution. Narrow spectral band high NA configurations, on the other hand, are useful in applications that require submicron resolution.

The variable NA capability in different n 34 nm × 26 nm step and scan fields offers a number of advantages in performing mix and match lithography. For example, the advantages of this multiple aspect of the optical system of the present invention allow different mask levels with features with different sizes to be imaged at selective resolution and emissivity levels rather than performing an exposure step on a number of different fixed parameter systems. Can be.

2-4 are diagrams of exemplary embodiments of the unit magnification projection optical system 100 of the present invention. This projection optical system 100 comprises a concave spherical mirror M1 along the axis A1. In an exemplary embodiment, the mirror M comprises an opening AP on the optical axis. This aperture (AP) may be used to provide light to the optical system to perform functions other than the ability to directly image into the optical system 100, such as aligning its image with an object (eg, a mask) or inspecting the object. Used to introduce inside.

The optical system 100 further comprises a variable aperture stop AS2 located in the mirror M. FIG. This variable aperture stop AS2 has any known form of varying the size of the aperture in the optical system, such as an adjustable iris. In one exemplary embodiment, the size of the variable aperture stop AS2 is set manually. In another exemplary embodiment, variable aperture stop AS2 is operably connected to controller 102 via line 101, which automatically sets the size of this aperture stop. The optical system 100 further comprises a field corrector (eg main) lens group G, spaced adjacent to the mirror M along the axis A1 and having a positive refractive power. This main lens group G comprises a positive subgroup GP farthest from the mirror M and a negative subgroup GN adjacent to the mirror M, which negative subgroup is relatively small. It is separated from the positive subgroup by the air filling space 104. This air fill space 104 further reduces residual aberrations, especially color deviations, while maintaining high quality imaging performance over large fields in the case of wide-spectrum band variable NA configurations and narrow-spectrum band variable NA configurations.

With continued reference to FIGS. 2 to 4, the adjacent positive groups GP are the first prism PA having the surfaces S1A, S2A and the second prism PB having the surfaces S1B, S2B. Surface S1A faces object plane OP2 and surface S1B faces image plane IP2. The object plane OP2 and the image plane IP2 are spaced apart from the respective flat surfaces S1A, S1B by respective gaps WDA, WDB representing the working space. In an exemplary embodiment where there is perfect symmetry with respect to the variable aperture stop AS2, WDA = WDB. Since WDA = WDB, these distances are referred to as WD in Tables 1-9.

Although prisms PA and PB are not included in the main lens group G, these prisms play an important role in aberration correction including chromatic aberration correction.

In the exemplary embodiment, the mirror M is unsphered to improve design performance in the case of large field high NA applications. All exemplary embodiments of the system of the present invention essentially retain system symmetry for the variable aperture stop AS2, which inherently eliminates odd order aberrations such as coma, distortion, and lateral collars. The optical system 100 comprises a lens surface concentric with the concave mirror M or with an asymmetric lens element in the main lens group G.

A major design improvement of the optical system 100 of the present invention over the prior art optical system of patent '494 is to selectively add an air fill space 104 and add a meniscus lens element and within the main lens group G. At least one non-spherical surface is optionally used. Major design modifications include field aberrations (especially astigmatism), spherical aberration, petzval curvature, axial color, and color deviation of these aberrations from different NAs to large field sizes and broad and narrow spectral bands. Suitably selecting an optical glass type for the field corrector lens (lens L1, L2 in FIG. 2 or lenses L1 to L3 in FIGS. 3 and 4) and prism therein for correction over.

Two-Element Main Group

With reference to FIG. 2, in certain exemplary embodiments, the positive subgroup GP comprises a single flat convex lens L1 and the negative subgroup GN comprises a single negative meniscus lens L2. In addition, in this embodiment, the lens elements L1, L2, together with the first and second prisms PA, PB, are used to collectively correct chromatic aberration over the optical spectral band including the g, h, i lines. It consists of two different glass types. A design based on this embodiment is proposed in Table 3.

Tables 1 and 2 propose a design based on this embodiment in which chromatic aberration is corrected over a narrow spectral band comprising i lines for variable NA and large fields of 0.34> NA> 0.16.

Three element main lens

In one exemplary embodiment, the main lens group G consists of three lens elements L1, L2, L3. Designs based on this embodiment are proposed in Tables 4-9 and are shown in FIGS. 3 and 4.

Together with the first and second prisms, the lens elements L1, L2, L3 comprise i lines in the case of variable NA and large field systems of 0.34> NA> 0.16 as in the designs proposed in Tables 4 and 5 Chromatic aberration is corrected over a narrow spectral band.

In one exemplary embodiment, the lens elements L1, L2, L3 have different glass types. With the first prism and the second prism, the chromatic aberration is corrected over the narrow spectral band. This exemplary embodiment is shown in the designs proposed in Tables 8 and 9.

In other embodiments, the glass types of the lens elements L1, L3 are the same, as in the designs proposed in Tables 6 and 7.

Single Element GP, Dual Element GN

Referring to FIG. 3, in one exemplary embodiment, the positive subgroup GP comprises a single flat convex lens L1, and the negative subgroup GN is three negative megaphones that are in optical contact or are bonded to each other. A negative doublet consisting of the varnish lens elements (L2, L3).

Single Element GN, Dual Element GP

Referring to FIG. 4, in one exemplary embodiment, the positive subgroup GP consists of a doublet with negative lens elements L1, L2, and the negative subgroup GN consists of a single lens L3. do. Also, in the present embodiment, the lens L1 is a flat iron lens element that is in optical contact or coupling with the negative meniscus lens element L2, and the lens L3 is a single negative meniscus lens element.

Example design

Other exemplary embodiments of the projection optical system 100 are apparent from the designs provided in Tables 1-9 and are described in more detail below.

If the optical system 100 is symmetric, the details proposed in Tables 1 to 9 only include values from the object plane OP2 to the concave mirror M. FIG. In Tables 1-9, the positive radius indicates that the center of curvature is to the right of the surface and the negative radius indicates that the center of curvature is to the left of the surface. The thickness of the element or the separation distance between the elements is the axial distance to the next surface and all sizes are within a few millimeters. In addition, "S #" represents a surface number as labeled in FIGS. 2 to 4, "T or S" represents "thickness or degree of separation", and "STOP" represents an opening stop AS2. In addition, "CC" shows a concave characteristic, and "CX" shows a convex characteristic.

Further, in the "surface shape" of the preceding part, the non-spherical surface is represented by "ASP", the flat surface is represented by "FLT", and the spherical surface is represented by "SPH".

A non-spherical equation representing a non-spherical surface is given by

Z = (CURV) Y ² / (1+ (l- (l + K) (CURV) ² Y ² ) ^1/2 + (A) Y ⁴ + (B) Y ⁶ + (C) Y ⁸ + (D ) Y ¹⁰ + (E) Y

Where "CURV" is the spherical curvature of the surface, K is the conic constant, and A, B, C, D, and E are non-spherical coefficients. In the accompanying tables, "e" represents exponential notation (power of 10).

Referring to Tables 1-9, in one exemplary embodiment, the optical system 100 has a three element main lens group G, wherein the first prism and the second prism PA, PB are the same as the Ohara BSM51Y. It is formed of glass type 603606 (see Table 8). Also, in the present embodiment, the flat iron lens L1 is formed of glass type 458678 such as fused silica or silica glass, and the meniscus lens elements L2, L3 are formed of glass type 581408 such as Ohara PBL25Y and Ohara PBL6Y. The same glass type is formed of 532490.

In another embodiment having a similar design configuration, lens L1 is formed of glass type 516643 such as Ohara BSL7Y or Schott BK7HT, BK7, UBK7, and lens L2 is formed of glass type 596393 such as Ohara PBM8Y (Table 9). The glass type for the lens L3 and prism in Table 9 is the same as used in the embodiment of Table 8.

In another exemplary embodiment, the optical system 100 comprises a three element main lens group G and the first prism and the second prism PA, PB are formed from a glass type 589612 such as Ohara BAL35Y (Table 5 Reference). Further, in the present embodiment, the flat iron lens L1 is formed of glass type 516643 such as Ohara BSL7Y or Schott BK7HT, BK7, UBK7, and the meniscus lens elements L2, L3 are formed of glass type 596393 such as Ohara PBM8Y and It is formed of glass type 458676, such as fused silica or silica glass.

In another exemplary embodiment, as shown in the designs proposed in Tables 8 and 9, optical system 100 comprises a three element main lens group G and lens L3 is on concave surface S7. Non-spherical surfaces. In an embodiment related to this, the lens L3 comprises a spherical surface on the concave surface S7 and the convex surface S8 as shown in Tables 5 and 6.

In another exemplary embodiment, as shown in the designs proposed in Tables 4-6, the optical system 100 comprises a three element main lens group G, and the lens L2 is on the surface S5. To have a spherical surface. In a similar embodiment, the lens element L1 has a non-spherical surface as shown in the designs proposed in Tables 1-3.

In another exemplary embodiment, the optical system 100 comprises a two or three element main lens group G and its field size comprises at least three 34 mm × 26 mm step and scan fields at 0.34 NA, At 0.26 NA, there are at least four 34 mm by 26 mm step and scan fields, and at or below 0.16, there are at least six 34 mm by 26 mm step and scan fields. Exemplary embodiments with this design are proposed in Tables 1, 4, 5, 6, and 7.

In another exemplary embodiment, the optical system 100 includes a two element main lens group G and its field size includes at least five 34 mm × 26 mm step and scan fields at 0.26 NA, and is no greater than 0.16. The NA contains at least six 34 mm × 26 mm step and scan fields (see Table 2).

In another exemplary embodiment, the optical system 100 comprises a two element main lens group G, the field size of which is of a broad spectral band application having a wavelength comprising the g, h, i spectral lines of the mercury light source. In the case of 0.26 NA, it contains at least four 34 mm by 26 mm step and scan fields, and in an NA below 0.16 it contains at least six 34 mm by 26 mm step and scan fields (see Table 3).

In another exemplary embodiment, the optical system 100 comprises a three element main lens group G whose field size comprises at least two 34 mm × 26 mm step and scan fields at 0.365 NA, and at 0.34 NA At least three 34 mm × 26 mm step-and-scan fields, including at least four 34 mm × 26 mm step-and-scan fields at 0.26 NA, and at least six 34 mm × 26 mm step-ends at NA up to 0.16 Include scan fields (see Tables 8 and 9).

Photolithography system

5 is a schematic diagram of a photolithography system 200 using a unit magnification projection optical system 100 according to the present invention. The system 200 includes a mask stage 210 having an optical axis A2 and supporting the mask 220 in the object plane OP2 along this optical axis. Mask 220 has a pattern formed on mask surface 226. An irradiator 230 is disposed adjacent the mask stage 220 opposite this optical system 100 and irradiates the mask 220.

The system 200 also includes a wafer stage 240 that supports moving the wafer 246 in the image plane IP2. In one exemplary embodiment, wafer 246 is coated with a photosensitive layer 250 that is activated by light having one or more wavelengths emitted from the irradiator. Such radiation is referred to in the art as "actinic radiation". In one exemplary embodiment, one or more wavelengths of such radiation include mercury g, h and i lines.

In operation, the irradiator 230 illuminates the mask 240 so that the pattern 224 is imaged at the wafer 246 by the optical system 100 to form a pattern in the photoresist layer 250. As a result, a part of the wafer becomes the exposure field EF. The wafer stage 240 then moves (steps) the wafer 246 in a predetermined direction (eg, in the x direction indicated by arrow 264) by a predetermined incremental distance (eg, by the size of one exposure field EF). . This exposure process is repeated. This step-and-expose process is repeated until the desired number of exposure fields EF are formed on the wafer 246 (ie, step and repeat process).

The wafer is then removed from the system 200 (using a wafer processing system not shown) and subjected to processes such as development, curing, etching, etc., to transfer the pattern formed in the photoresist in each scanned exposure field EF to the wafer. . By repeating this photolithography process with a different mask, a three-dimensional structure is formed in the wafer to create an operating device such as an integrated circuit. In addition, by adjusting the NA of the system 100, the scanned field EFs having different sizes and different resolutions can be set to correspond to a predetermined mask, thereby simplifying the mix and match lithography process. .

In the foregoing Detailed Description, various features are grouped together in various embodiments for ease of understanding. Numerous features and advantages of the invention have been made apparent in the detailed description, and the appended claims cover all of the above features and advantages of the described apparatus which follow the spirit and scope of the invention. In addition, many modifications and variations are possible to those skilled in the art, and the invention is not to be limited precisely to the configurations and operations described herein. Accordingly, other embodiments are included within the scope of the appended claims.

Claims (34)

In an optical system, Along the optical axis, With concave spherical mirror, A variable aperture stop located in the concave spherical mirror and determining a numerical aperture (NA) of the optical system, A first subgroup having positive refractive power and positioned adjacent to the concave spherical mirror, spaced apart from the mirror, having a positive refractive power in order towards the mirror and having a negative refractive power and by means of an air layer A main lens group including a second subgroup spaced apart from the subgroup, A first prism and a second prism each having a first flat surface and a second flat surface, respectively, The second flat surfaces are disposed adjacent to the positive subgroup on opposing sides of the optical axis, the first flat surfaces are respectively disposed adjacent to the object plane and the image plane, The optical system having an adjustable exposure field in the image plane by adjusting the variable aperture stop to image two or more 34 mm by 26 mm step-and-scan fields. The method of claim 1, The adjustable exposure field is At 0.365 NA, it contains at least two 34 mm × 26 mm step-and-scan fields, At 0.34 NA, it contains at least three 34 mm × 26 mm step and scan fields, At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 1, The adjustable exposure field is At 0.34 NA, it contains at least three 34 mm × 26 mm step and scan fields, At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 1, The system is aberration corrected over a spectral band comprising g-line (436 nm), h-line (405 nm) and i-line (365 nm), The adjustable exposure field is At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 1, The adjustable exposure field is At 0.26 NA, it contains five or more 34 mm × 26 mm step-and-scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 1, The system is aberration corrected over a narrow spectral band including i-line, The adjustable exposure field is At least 0.36 NA comprising at least two 34 mm by 26 mm step and scan fields. The method of claim 1, Optical system having an adjustable NA between 0.34 and 0.16. The method of claim 1, Optical system having an NA adjustable from O.365 to 0.16. The method of claim 1, And a controller operatively connected to the variable aperture stop to automatically adjust the variable aperture stop. The method of claim 1, At least one lens element in the primary lens group comprises an aspheric surface. The method of claim 1, The mirror has an aspheric surface. The method of claim 1, And the first prism and the second prism are each formed from one of glass type 603606 and glass type 589612. In an optical system, Along the optical axis, With concave spherical mirror, A variable aperture stop positioned in said concave spherical mirror and determining a numerical aperture NA of said optical system, A first planar lens having a positive refractive power and positioned adjacent said concave spherical mirror, spaced apart from said concave spherical mirror, and having a convex surface facing said mirror in order towards said mirror; A second subgroup consisting of a single negative meniscus lens having a subgroup and a convex surface facing the mirror and having a concave surface spaced apart by an air layer from the convex surface of the flat iron lens. The main lens group to include, A first prism and a second prism each having a first flat surface and a second flat surface, respectively, The second flat surfaces are disposed adjacent to the plane of the flat lens on opposing sides of the optical axis, the first flat surfaces are respectively disposed adjacent to the object plane and the image plane, The optical system having an adjustable exposure field in the image plane by adjusting the variable aperture stop to image two or more 34 mm by 26 mm step and scan fields. The method of claim 13, And the flat iron lens consists of one of glass type 458678 and glass type 516643. The method of claim 13, Each of the prisms is formed from one of glass type 603606 and glass type 589612. The method of claim 13, The adjustable exposure field is At 0.34 NA, it contains at least three 34 mm × 26 mm step and scan fields, At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 13, Optical system having an adjustable NA between 0.34 and 0.16. The method of claim 13, At least one lens element in the primary lens group comprises an aspheric surface. The method of claim 13, The mirror has an aspheric surface. The method of claim 13, The first prism and the second prism are formed of glass type 603606, The flat iron lens is formed of a glass type 458678, The meniscus lens element is formed of one of the glass types 548458, 567428, 581408,596393 and 596387. In an optical system, Along the optical axis, With concave spherical mirror, A variable aperture stop positioned in said concave spherical mirror and determining a numerical aperture NA of said optical system, A first subgroup consisting of a flat iron lens having a positive refractive power and positioned adjacent said concave spherical mirror, spaced apart from said concave spherical mirror, and having a convex surface facing said concave spherical mirror in order towards said concave spherical mirror; A second subgroup consisting of a first negative meniscus lens and a second negative meniscus lens each having a convex surface facing the concave spherical mirror, wherein the concave surface of the first negative meniscus lens comprises: A main lens group spaced apart from the convex surface of the flat iron lens by an air layer, A first prism and a second prism, each having a first flat surface and a second flat surface, wherein the second flat surfaces are disposed adjacent to the plane of the flat iron lens on opposite sides of the optical axis, The first flat surfaces comprise a first prism and a second prism, respectively disposed adjacent to the object plane and the image plane, The optical system having an adjustable exposure field in the image plane by adjusting the variable aperture stop to image two or more 34 mm by 26 mm step and scan fields. The method of claim 21, Each of the flat iron lens and the meniscus lens together with the first prism and the second prism g-line (436 nm), h-line (405 nm) and i-line (365 nm) for NA <0.26. And collectively correct chromatic aberration over a broad spectral band, including N, and narrow spectral bands covering an i-line (365 nm) for NA> 0.26. Optical systems having different glass types from which to correct as a rule. The method of claim 21, The adjustable exposure field is At 0.365 NA, it contains at least two 34 mm × 26 mm step-and-scan fields, At 0.34 NA, it contains at least three 34 mm × 26 mm step and scan fields, At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 21, At least one lens element in the primary lens group comprises an aspheric surface. The method of claim 21, The mirror has an aspheric surface. The method of claim 21, One of the meniscus lenses and the flat iron lens has the same glass type. In an optical system, Along the optical axis, With concave spherical mirror, A variable aperture stop located in the mirror and determining the numerical aperture (NA) of the system; A main lens group having a positive refractive power and positioned adjacent the mirror, spaced apart from the mirror, the primary lens group including a first subgroup and a second subgroup in order towards the mirror, the first subgroup facing the mirror A second double group consisting of a flat iron lens having a convex surface facing the mirror in order and a first negative meniscus lens element having a convex surface facing the mirror; A main lens group composed of a second negative meniscus lens having a convex surface facing the mirror and having a concave surface separated by the convex surface of the first negative meniscus lens by an air layer; A first prism and a second prism, each having a first flat surface and a second flat surface, wherein the second flat surfaces are disposed adjacent to the plane of the flat iron lens on opposite sides of the optical axis, The first flat surfaces comprise a first prism and a second prism, respectively disposed adjacent to the object plane and the image plane, The optical system having an adjustable exposure field in the image plane by adjusting the variable aperture stop to image two or more 34 mm by 26 mm step and scan fields. The method of claim 27, The adjustable exposure field is At 0.365 NA, it contains at least two 34 mm × 26 mm step-and-scan fields, At 0.34 NA, it contains at least three 34 mm × 26 mm step and scan fields, At 0.26 NA, it contains at least four 34 mm × 26 mm step and scan fields, An optical system comprising at least six 34 mm by 26 mm step-and-scan fields at NA of 0.16 or less. The method of claim 27, At least one lens element in the primary lens group comprises an aspheric surface. The method of claim 27, The mirror has an aspheric surface. The method of claim 27, Both the first prism and the second prism are formed from one of glass type 603606 and glass type 589612. The method of claim 31, wherein And the flat iron lens is formed from one of glass type 458678 and glass type 516643. The method of claim 32, One of the meniscus lens elements is formed of a glass type selected from a glass type group consisting of glass type 548458, 567428, 581408,596393, 532490, and 596387. In a projection lithography system, Optical system, A mask stage that supports the mask in the target plane, an illuminator for irradiating the mask with at least one of a g-line wavelength, an h-line wavelength, and an i-line wavelength; A wafer stage for moving and supporting the wafer in the image plane, The optical system, Along the optical axis, With concave spherical mirror, A variable aperture stop positioned in said concave spherical mirror and determining a numerical aperture of said optical system; A first subgroup having positive refractive power and positioned adjacent to the mirror but spaced apart from the mirror, the first subgroup having positive refractive power in order toward the mirror and having a negative refractive power and spaced apart from the first subgroup by an air layer A main lens group including subgroups, A first prism and a second prism, each having a respective first flat surface and a second flat surface, the second flat surfaces being disposed adjacent to the positive subgroup on opposing sides of the optical axis, The first flat surfaces comprise a first prism and a second prism, respectively disposed adjacent to the object plane and the image plane, And the optical system has an adjustable exposure field in the image plane by adjusting the variable aperture stop to image two or more 34 mm by 26 mm step and scan fields.